1. Field
This disclosure relates to reflectors for microwave and millimeter wave radiation.
2. Description of the Related Art
A passive reflect array is an array of conductive elements adapted to reflect microwave or millimeter wave radiation within a predefined wavelength band. The array of conductive elements is typically separated from a continuous ground plane by a thin dielectric layer such that the incident microwave or millimeter wave radiation is reflected by the combined effect of the ground plane and the conductive elements. Since the incident radiation may be reflected with a phase shift that is dependent on the size, shape, or other characteristic of the conductive elements, the term “phase-shifting element” will be used to describe the conductive elements of a reflect array.
The size, shape, or other characteristic of the phase-shifting elements may be varied to cause a varying phase shift across the extent of the array. The varying phase shift may be used to shape or steer the reflected radiation. Reflect arrays are typically used to provide a reflector of a defined physical curvature that emulates a reflector having a different curvature. For example, a planar reflect array may be used to collimate a diverging microwave or millimeter wave beam, thus emulating a parabolic reflector.
Reflect arrays which include crossed-dipole phase-shifting elements are described in U.S. Pat. No. 4,905,014. FIG. 8 shows a graph 800 of data, obtained by simulation, showing the performance of a cross-dipole reflect array as a function of the dipole length dimension Ldipole for normally-incident radiation. The data summarized in the graph 800 was simulated for a frequency of 95 GHz using specific assumptions for the substrate material, substrate thickness, grid spacing Dgrid, and dipole width Wdipole. In FIG. 8 (and FIGS. 3, 5, and 6 to be subsequently described), the plotted phase shift is defined as the phase difference between a simulated incident wavefront and a reflected wavefront, both measured at a reference plane displaced from the surface of the reflect array. Thus the phase shift data contains a constant phase offset due to the round trip propagation from the reference plane to the reflect array and back.
As shown by the curve 810, the phase shift may be varied from about +105 degrees to +156 degrees (after wrapping through ±180 degrees) by varying the dipole length from less than 10 mils (0.010 inches) to more than 70 mils (0.070 inches). However, for the assumed combination of substrate material, substrate thickness, grid spacing Dgrid, and dipole width Wdipole, it is not be possible to achieve a phase shift between +156 degrees and +105 degrees, leaving a “gap” of about 51 degrees. The inability to achieve a continuously variable phase shift over a 360-degree range may limit the capability of a reflect array to accurately direct and form a reflected beam.
As shown by the dashed curve 820, the simulated reflection loss also varies with the dipole length. The reflection loss curve shows a single peak, at a dipole length about 0.042 inch, due to a resonance within the phase-shifting elements. For a crossed-dipole reflect array, the reflection loss peak may occur when the dipole length is equal to one-half of the wavelength of the reflected radiation (including the effect of the dielectric constant of the substrate). The reflection loss peak may occur when the length of the dipole is such that the dipole resonates at the wavelength being reflected from the reflect array. As shown by the solid curve 810, the dependence of phase shift on the dipole length is strongest in the vicinity of the resonance. The phase shift varies substantially when the dipole length is varied from about 0.03 inch to about 0.05 inch, but is relatively constant for dipole lengths less than about 0.03 inch or greater than about 0.05 inch.